Shape Key decimate modifier
Shape keys in blender allows you to translate and  scale different vertices, edges and faces of a mesh. Even though Shape keys changes the structure/geometry of a mesh, they always maintain the same number of polygons.

So any changes involving changes in poly count usually won't work well for Shape Keys. So for this situation, Shape Keys cannot be used.

However,you could try animating the iterations/ratio/angle limit of the decimate modifier. Just press i on a slider and the slider turns yellow which means the slider is Keyframed.